The invention is directed to an aqueous adhesive composition as well as to a method for adhering a first substrate to a second substrate.
Various types of adhesive compositions are known that are suitable for adhering various substrates, such as paper and board.
Adhesives may be divided in groups by their method of adhesion. For example, drying adhesives (which can be further subdivided into solution and emulsion adhesives) and hot melt adhesives are two different groups of adhesives known in the art.
Drying adhesives set by drying. Drying increases the viscosity of the adhesives and increases the bond strength between two substrates glued with the adhesive. Drying adhesives are preferably water-based (waterborne). Drying adhesives can further be subdivided into adhesives wherein the adhering polymer is dissolved (solution adhesives) and polymer dispersion adhesives (emulsion adhesives).
Solution adhesives typically comprise hydrophilic polymers, such as natural polymers (for example starch, dextrin, cellulose derivatives, casein) or soluble synthetic polymers (for example polyvinyl alcohol), which polymers are dissolved in water. As the solvent evaporates, the adhesive sets. Due to their high affinity for water, the polymers in a solution adhesive release water very slowly. As a result, solution adhesives are generally relatively slow setting and take a longer drying period compared to emulsion adhesives. Solution adhesives are particularly suitable for the adhesion of porous substrates. The porosity of the substrate facilitates the diffusion of the solvent from solution adhesives, thereby enhancing drying speed.
Emulsion adhesives typically comprise a polymer dispersed in water. An emulsion adhesive comprises hydrophobic polymers, such as polyvinyl acetate (PVAc), dispersed in water. Emulsion adhesives may contain less solvent than solution adhesives, because the polymer is not dissolved, but merely dispersed and the water is repelled by the hydrophobic nature of the polymer. As a result, emulsion adhesives generally set faster compared to solution adhesives. Emulsion adhesives typically have a high solid content. Emulsion adhesives are frequently used in such industries as the woodworking and packaging industries.
An advantage of drying adhesives containing non-thermoplastic polymers such as natural carbohydrates is that they are not particularly sensitive to temperature. Such drying adhesives are able to provide a good bonding strength between two glued substrates at both low and high temperatures.
A further advantage of drying adhesives, in particular waterborne drying adhesives, is that they can often be environmentally friendly.
A further property of drying adhesives is that they set relatively slowly. This may to a certain degree be advantageous, because it allows for a certain assembly time of the two substrates to be glued. However, the setting time of drying adhesives, in particular waterborne drying adhesives, is typically long and may even be so long that it may give insufficient bonding in fast applications. Drying adhesives require one of the substrates to be water permeable to allow for drying to occur.
A further disadvantage of drying adhesives, in particular waterborne drying adhesives, is that they are typically vulnerable to microorganisms and/or may have poor moisture resistance.
Hotmelt adhesives (hotmelts) are adhesives that are solid at room temperature and become a fluid at elevated temperature. When a hotmelt adhesive is heated above its softening point, typically at 140-180° C., it becomes fluid and can be applied to a substrate. Upon cooling below the softening point the hotmelt solidifies and may bind two substrates together.
Some materials, such as plastics and gels, do not have a definite melting point. For such materials, a so-called softening point may be used to indicate the material's transition temperature from a solid or gel state to a fluid state. The softening temperature can be determined, for example, by the ring & ball method (ASTM E28-99), the Vicat method (ASTM-D1525 or ISO 306) or the Heat Deflection Test (ASTM-D648). The softening point is preferably determined by the ring & ball method. The softening point as determined by the ring & ball method is the temperature at which a standardized steel ball cannot be held by a disk of the material anymore. This method of determining the softening point is described in more detail in the experimental section below.
Hotmelts are typically thermoplastic adhesive compositions. A thermoplastic is an organic material that can be repeatedly softened by heating and hardened by cooling. Hotmelt adhesives are used in many branches of the adhesive industry, in particular in the packaging and bookbinding industry.
The advantage of hotmelt adhesives is that they typically provide for short setting times, e.g. below 5 seconds, such that the substrates need only be pressed together for a short time after glueing (a short pressing time). At the end of the pressing time, the adhesive strength should be high enough to hold the substrates together. Furthermore, hotmelts are able to bond many different types of substrates, including permeable substrates and non-permeable substrates.
A disadvantage of hotmelts is that they are inherently temperature sensitive. Hotmelts become soft at elevated temperatures and become brittle at low temperatures. At elevated temperatures hotmelts are susceptible to joint movement or creep, which can result in bond failure.
A further disadvantage of hotmelts is that they typically have a high application temperature of between 140-180° C. Such high temperatures may be undesirable with respect to energy consumption and operator safety and may further be undesirable to use in certain applications, for example when one of the substrates to be glued is a heat-sensitive substrate.
A further disadvantage of hotmelts is that the maximum amount of time between applying the adhesive on one substrate and application of the second substrate at which still a good bond is achieved (“open time”) is short, which makes the time available for part adjustment limited. Part adjustment is the operation of adjusting the two substrates relative to each other. The operation of part adjustment is possible only for as long as the glue between the substrates has not fully set yet and thus allows for moving the substrates relative to each other. In order to increase the open time often more adhesive is applied than needed to achieve a suitable bond strength (see Irving Skeist, Chapter 23: Polyolefin and Ethylene Copolymer-based Hot Melt Adhesives, p 408-p 422, Handbook of Adhesives, Third Edition, 1989, Chapman & Hall). This solution is not desirable for obvious reasons.
Plasticizers have been used extensively to obtain hotmelts based on water soluble polymers, such as starch, which are not inherently thermoplastic. In particular, low molecular weight, water soluble compounds have been used as plasticizers, such as illustrated in U.S. Pat. No. 5,454,862, WO 92/19690 and GB-A 964 799.
A disadvantage of using plasticizers in such hotmelts is that plasticizers have a tendency to migrate under the influence of moisture, resulting in bleeding (visible smudging of the surface of the substrate) and bond failure due to brittleness and loss of cohesion.
Although nowadays hotmelts are solvent free systems based on synthetic polyolefins, the early hotmelt adhesives were based on ethyl cellulose and animal and hide glues and were waterborne. These were abandoned because they were suffering from disadvantages such as being unable to cope with higher machine speeds or having too low softening point and bad smell. The thermoplast based hotmelts provided 100% solid adhesives requiring no water transport, having no smell, and adhering to more diverse substrates.
U.S. Pat. No. 5,589,528 describes a hot melt adhesive based on modified water bearing gels having a softening point below approximately 43° C. The hot melt adhesive may comprise 50-70% natural organic material content (such as animal hide and bone, fish, blood and casein, soy, starch, cellulosics, etc.), 25-50 wt. % water. The hot melt adhesive may further comprise sugars and tackifiers, for example starch components.
A disadvantage of the composition described in U.S. Pat. No. 5,589,528 is that the softening point of the hotmelt is too close to room temperature, which will result in slow setting of the adhesive at room temperature. A further disadvantage is that the manufacture of the hotmelt is laborious and conducted in multiple steps.
WO 98/15347 is directed to a modified starch obtainable by treating amylose-containing starch in aqueous medium with an enzyme having α-1,4-α-1,4-glucosyl transferases activity. Such a modified starch can be used as an agent for forming a thermoreversible gel. The modified starch is suitable for many uses in which the property to form a thermoreversible gel can be useful or important, such as in foodstuffs, cosmetics, pharmaceutics, detergents, adhesives and drilling fluids. WO 98/15347 further describes an aqueous solution comprising 5% potato starch modified with glucosyl transferase, which solution shows the behaviour of a thermoreversible gel. Disadvantage of the composition of WO 98/15347 is that its gelling speed is slow. It takes hours for the gel to form, even at low temperatures such as 5° C. This makes the thermoreversible gel of WO 98/15347 unsuitable as an adhesive.
Another approach to obtain a thermoreversible gelling starch is provided in EP 0 372 184. In the process described, starch is partially debranched using enzymes with debranching activity. The starches obtained consist of a mixture of short chain amylose, partially debranched amylopectin and amylopectin in ratios depending on starting materials and processing conditions. The formation of gels using these starches is also too slow to be suitable as an adhesive.